1. Field of the Invention
This invention relates generally to slide plate screen changers of the type used in removing impurities from flowable materials such as for example extruded polymers. The invention is concerned in particular with an improved arrangement for adjusting the seals between which the slide plates are confined or sandwiched to prevent leakage of the material being filtered.
2. Description of the Prior Art
Conventional slide plate screen changers include a housing defining a flow passage for the material to be filtered, with a slide plate carrying a plurality of filter screens, the slide plate being movable transversally across the flow passage to expose selected ones of the filter screens to the material being filtered. Seal rings surround the flow passage on the upstream and downstream sides respectively of the slide plate to prevent escape of the material being filtered. The downstream seal ring is usually stationary, with the upstream seal ring being adjustable towards the downstream seal ring to tightly confine the slide plate therebetween. In one known arrangement, this adjustment is accomplished by directly engaging the upstream seal ring with an adjusting nut which surrounds the flow passage and which is threaded into the housing at a location at least partially confined within the housing intermediate its upstream and downstream ends. One problem with this type of arrangement is that polymers have a tendency to seep past the upstream seal ring into the threads of the adjusting nut, often filling clearances between the nut and the housing. This polymer then degrades, jamming the threads and making it extremely difficult to back the nut off when replacing worn seals. Also, by confining the adjusting nut within the housing between its upstream and downstream ends, access to the nut is limited, and the overall size of the housing is increased unnecessarily.
Attempts have been made at overcoming the above problems by replacing the confined adjusting nut with an external flanged ring attached to the upstream side of the housing by a plurality of adjusting bolts. The problem with this arrangement, however, is that it is difficult if not impossible to uniformly tighten the individual adjusting bolts in order to obtain optimum seating of the upstream seal ring against the slide plate. Also, individual adjustment of the bolts is tedious, and unnecessarily time consuming.